Re: tire size / RPM connection

[Carter Shore <clshore@yahoo.com>]

Jeff,
Calculate your speed with this formula:

   d * RPM
------------- =   speed, MPH
 336 * ratio

where:
d = tire diameter in inches ( 2X rolling radius )
RPM = revs per minute (duh)
ratio = rear end ratio, 4.11, 4.55, 3.27, etc.

So, if your tire was 25" dia, and you had a 4:88 rear
end, you would be  going about 61 MPH.

BTW, my rule of thumb in a Spitfire with 4:11 gears is
20 MPH per 1000 RPM. (from driving around without a
speedo for about a year).

Carter